1. Field of the Invention
The present invention relates to an injection molding machine and, more particularly, to an injection molding machine including a mold for molding disk-shaped recording medium carriers such as optical disk substrates (hereinafter abbreviated to "optical disk substrates").
2. Description of the Prior Art
A conventional mold for molding optical disk substrates includes an ejector rod for ejecting a molded substrate, a gate-cutting punch, and an ejector pin for ejecting a sprue. The substrate-ejecting ejector rod is movably fitted in the main body of the mold while facing the cavity. The gate-cutting punch and the sprue-ejecting ejector pin are movably fitted within the ejector rod while concentrically engaging with each other in such a manner as to be movable relative to each other. The conventional mold also incorporates therein a pneumatic cylinder to actuate mechanisms for ejecting the substrate and a hydraulic cylinder to actuate a gate-cutting mechanism.
However, because the conventional mold incorporates hydraulic cylinders, various problems arise. The structure of the mold is complicated. During the manufacture of the mold, considerations have to be given to various mechanisms including the mechanism for sealing, e.g., high-pressure hydraulic fluid, and the mechanism for assuring pressure resistance. In addition, a great number of component parts are required. Accordingly, it is an inevitable consequence that a mold for molding optical disk substrates is relatively large, heavy, and expensive. Further, proper maintenance of the mold is difficult. Still further, when the sealing of the pressure oil happens to be inadequate, there is a risk of oil leakage, this being undesirable particularly in the case of optical disk substrates which must not be contaminated. Another problem is that, during the replacement of the mold, since the component parts that must be connected through pipe lines include not only a temperature adjusting device but also the gate-cutting hydraulic cylinder, the operation of connecting pipe lines is inevitably laborious.